1. Technical Field
The present invention relates to an image forming apparatus such as a printer, a facsimile, or a copying machine for forming an image on a recording medium such as paper by using the electronic photography technique and; more particularly, to an image forming apparatus having a secondary transfer device for transferring a toner image by applying a transfer bias voltage from an intermediate transfer member to a recording medium under a constant current control.
2. Background Art
Japanese Patent Publication No. 11-288184, discloses an image forming apparatus for forming an image on a recording medium such as a paper sheet by using the electronic photography technique. This image forming apparatus has a secondary transfer device for applying a transfer bias voltage to a secondary transfer roller under a constant current control so that a toner image on an intermediate transfer member such as an intermediate transfer belt or an intermediate transfer drum may be secondarily transferred to the recording medium such as the paper sheet. In this apparatus, at the position opposing a drive roller (serving as a secondary transfer backup roller) for the intermediate transfer belt, there is disposed a secondary transfer roller which is retractably brought into contact with the intermediate transfer belt. At this position, the transfer bias voltage is applied to the secondary transfer roller by a constant current controller so that toner images of four colors on the intermediate transfer belt are secondarily transferred to the recording medium.
The recording medium having the transferred tone image is charged. If this charged state is left as it is, a drawback is encountered by the obstruction to the subsequent smooth transporting of the recording medium, a charge elimination has to be made on the recording medium. For this purpose, there is provided a charge eliminator, which is formed of non-woven fabric of conductive fibers into a sheet, in the vicinity of the downstream side of the secondary transfer roller. The charge eliminator is adhered to a conductive plate for applying a charge elimination voltage thereto, and eliminates the charge on the recording medium after the secondary transfer.
FIG. 2 shows an example of such a secondary transfer device. An intermediate transfer belt 21 is stretched between a drive roller 50 serving as a secondary transfer backup roller and a not-shown follower roller or tension roller, and is circulated in a prescribed direction by the drive roller 50. The secondary transfer roller 51 is retractably brought into contact with the intermediate transfer belt 21 by a not-shown actuating mechanism. When the secondary transfer roller 51 comes in contact with the intermediate transfer belt 21, it is pressed against the intermediate transfer belt 21 by a not-shown urging member such as springs mounted on the both ends of the secondary transfer roller 51. The secondary transfer roller 51 is rotated in cooperation with the circulation of the intermediate transfer belt 21, and a recording medium is transported into a nip portion formed between the secondary transfer roller 51 and the intermediate transfer belt 21 at a prescribed angle defined by a guide member 52 which is configured to be pivotable about a pivot shaft 53.
The guide member 52 is equipped with a charge eliminator 54 adjacent to the secondary transfer roller 51. The charge eliminator 54 is made of conductive non-woven fabric (or charge eliminating fabric), which is worked from numerous conductive fibers into a sheet shape. The upper face of the charge eliminator 54 made of the charge eliminating fabric is opposed to the recording medium having passed through the secondary transfer device so that a discharge is established between the charge eliminator 54 and the recording medium thereby to eliminate the charge on the recording medium. The charge eliminator 54 is adhered to a conductive plate 55. The charge eliminating voltage is applied from a not-shown power source through the plate 55.
As shown in FIG. 3, an attracting force (represented by an arrow A) by the plate 55 and a winding force (represented by an arrow B) by the winding electric field to occur between the intermediate transfer belt 21 and a recording medium S after the secondary transfer. The winding electric field is influenced by the environmental change of the secondary transfer roller 51 under the constant current control and by the resistance change due to the kind of the recording medium S. If the charge elimination or the recording medium S by the charge eliminator 54 is insufficient, the winding force between the recording medium S and the intermediate transfer belt 21 becomes stronger than the attracting force of the plate 55 thereby to cause a phenomenon that the intermediate transfer belt 21 is wrapped with the recording medium S after the secondary transfer. If the attracting force by the plate 55 is excessively strong, the recording medium S contacts the guide member 62 to cause a damage on the image. If the charge on the recording medium S is excessively eliminated, the force to attract the toner to the recording medium S is weakened to obscure the image.
For preventing the recording medium S from winding on the intermediate transfer belt 21 thereby to attain an image of high quality, it is necessary to balance the attracting force A by the plate 55 and the winding force B by the winding electric field to occur between the intermediate transfer belt 21 and the recording medium S, as shown in FIG. 4.
There are various causes for winding the recording medium on the intermediate transfer belt 21. One winding cause is the fluctuations of the secondary transfer voltage due to the environmental change at the time the secondary transfer by applying the secondary transfer bias voltage under the constant current control. Since the fluctuation ranges of the secondary transfer voltage are different for their causes, the charge elimination bias voltage (i.e., the charge elimination ability) has to be controlled finely. Moreover, frequency of the occurrence of the winding is different for the typo and thickness of the recording medium and for the order of first or second of the double-sided printing. Therefore, it is necessary to control the charge eliminating bias voltage considering not only the fluctuations of the secondary transfer voltage but also another winding cause. Otherwise, problems of the loss of power consumption and the shortened lifetime of the charge eliminator 54 are caused.
Another cause for the winding of the recording medium on the intermediate transfer belt 21 is the change in the durability of the charge eliminator 54. This is because the paper powder sticks to the charge eliminator 54 thereby to lower the charge eliminating ability as the operation time period increases.